Applications of advancing medical technology, such as cardiopulmonary bypass, are associated with a variety of complications. Even in short term use, blood oxygenators produce sufficient activation of clotting pathways to require the use of heparin to inhibit blood coagulation.
An invariable complication of such surgical procedures is a hemorrhagic state manifested by a prolonged bleeding time, causes of which include failure to adequately neutralize heparin and the continuous stimulation of platelets, as manifested by a fall in platelet count, stimulation of thromboxane synthesis and release of platelet granule constituents. (See Colman, J. Anesthesiology, vol. 66 (5), May, 1987, p. 595). Reversal of heparin is required to restore normal coagulation status and reduce post-operative blood loss.
Protamine is an arginine-rich polypeptide (32 amino acids from salmon) commonly used at the conclusion of cardiovascular surgical procedures to neutralize the anticlotting effects of heparin. The use of protamine, however, has been linked to several post-surgical complications, some of which are postoperative systemic hypotension, allergic reactions, catastrophic pulmonary vasoconstriction, acute pulmonary hypertension, complement activation, noncardiogenic pulmonary edema, decreased cardiac output (later event), and thrombocytopenia/leukopenia.
The underlying biochemical basis for these physical complications is poorly understood, but allergic reactions to protamine, have been well documented. Since protamine, usually isolated from fish, can be recognized as a foreign protein by the human immune system, patients with prior protamine exposure are at particular risk during subsequent exposures (Just Viera, J. O., Amer. Surgeon 50, (1984), 151-163). Additionally, studies suggest that a non-immunological pathway via complement activation may be responsible for many of the acute reactions observed during protamine reversal of heparin anticoagulation.
To avoid the use of protamine, a number of approaches have been proposed. Construction of bypass circuits with materials that do not activate the coagulation cascade have been suggested, as well as the use of non-heparin anticoagulate preparations. Neutralizing agents for heparin other than protamine are also currently being sought. Horrow, "Effective Hemostasis in Cardiac Surgery", chap. 2, Ellison and Jobes, Eds, in press, 1988. All of these alternatives, presently in various stages of research, have yet to reveal a suitable substitute for protamine that has gained widespread acceptance.
PF4 is a well characterized heparin-binding protein found in human plasma at concentrations of 5-20ng/ml. In plasma, in in vitro studies, PF4 has been demonstrated to reverse the effect of heparin on clot formation. Michalski, Brit. J. Haematol, 38, 561-571 (1978).
In vivo experiments demonstrate that PF4 is cleared rapidly from circulation in both rats and rabbits. This rapid clearance is due to PF4 binding to endothelial cells. PF4 bound by endothelial cell surfaces can be released into the bloodstream by subsequent administration of heparin. In humans, a similar rise in circulating PF4 concentration is observed following heparin administration.
Platelet concentrate has been shown to have heparin neutralizing activity when administered to humans after bypass surgery, and the effect was attributed to PF4. Walker, Br. Heart J. 52:12-14 (1984).